It is actually for a guitar but works equally well for a banjo. Go to my photos, I have posted a page from Justin Holland Guitar Method 1888 that shows the same capo design. Also his instructions for the "capotasto." The design may go back as far as the appearance of six string "Romantic Guitar" itself, which about 1780, give or take.

It's not a huge deal but it works slightly better if you replace the black nylon tension string with one of your old gut strings if you have one. The gut has a little spring in it. It will stretch a wee bit under high tension and the result is it will tighten better and hold (it has friction- it's gut). It also looks better. Pretty sure the capotastos in the 1800s did not use nylon! ;-)